1. Field of the Invention
The present invention relates to a method and a system for checking (diagnosing) the states of a plurality of external signal input/output units (I/O units) used when automated machines such as machine tools are controlled by a numerical controller.
2. Description of the Related Art
As is well known, when automated machines such as machine tools are controlled by a numerical controller, various external signals are input or output. An external signal input/output unit used to input or output such external signals is called an I/O unit. The external signals input to or output from an automated machine through an I/O unit include, when the automated machine is a machine tool for example, a start signal, a synchronization signal for each servo axis, a reset signal, an override signal, an emergency stop signal, and other signals. These signals are directly related to safety, so a fault that occurs in the I/O unit needs to be detected immediately.
A typical method conventionally used is to check whether the I/O unit is operating normally by periodically operating a self-diagnostic circuit provided in the I/O unit. FIG. 2 is a block diagram showing an overview of an I/O unit with a diagnostic circuit in which this method is applied.
In FIG. 2, the I/O unit A1 has a host transmitting/receiving block 11, which controls the exchange of signals with the numerical controller 1 (CNC) and with another I/O unit A2, an external signal transmitting/receiving block 12, and a diagnostic circuit 13. The external signal transmitting/receiving block 12 is connected to an output line 21 via a contact C1, an input line 22 via a contact C4, a diagnostic loop line 23 via a contact C2, and another diagnostic loop line 24 via a contact C3. The output line 21 and the input line 22 are connected to an automated machine M1 for which the I/O unit A1 provides input and output.
Of the contacts C1 to C4, the contact C1 connected to the external signal output line 21 and the contact C4 connected to the external signal input line 22 are normally closed and remain closed (electrically connected) unless commanded to open by the diagnostic circuit 13. These contacts C1 to C4 are opened (electrically disconnected) when a diagnosis start signal is output from the diagnostic circuit 13, and are closed (electrically connected) when a diagnosis end signal is output from the diagnostic circuit 13. The contacts C2 and C3 connected to the diagnostic loop lines 23 and 24 are normally open, are closed (electrically connected) when the diagnosis start signal is output from the diagnostic circuit 13, and are opened (electrically disconnected) when the diagnosis end signal is output from the diagnostic circuit 13.
In general, the I/O unit A1 is diagnosed periodically. For this purpose, the I/O unit A1 has a timer (not shown) set to trigger the diagnostic circuit 13 at certain intervals (for example, intervals of 10 minutes). First, the diagnosis start signal is output from the diagnostic circuit 13, opening contacts C1 and C4 and closing contacts C2 and C3. Next, the diagnostic circuit 13 transmits an output diagnostic signal (a prescribed digital signal packet) to the external signal transmitting/receiving block 12 and checks whether or not the transmitted signal is successfully returned to the diagnostic circuit 13 through the closed contact C2 and diagnostic loop line 23.
In addition, the diagnostic circuit 13 transmits an input diagnostic signal (another prescribed digital signal packet) to the external signal transmitting/receiving block 12 through diagnostic loop line 24 and the closed contact C3 and checks whether or not the transmitted signal is successfully returned to the diagnostic circuit 13. If a failure occurs during either input or output, the operator is informed by suitable means (for example, a beeper).
The input line 22 and output line 21 are connected to the automated machine M1 (for example, a machine tool) that is to be controlled, and when the unit is not being diagnosed, various external signals such as the servo axis synchronization signals, reset signal, override signal, and emergency stop signal are transmitted or received through the closed contacts C1 and C4. In the example shown in FIG. 2, the host transmitting/receiving block 11 is connected to the CNC 1 and the host transmitting/receiving block of another I/O unit A2, which has the same structure as I/O unit A1.
In a system in which a plurality of machine tools are controlled by one CNC, when, for example, I/O units A1 to A5 are connected to respective machine tools (more generally, automated machines) M1 to M5 as shown in FIG. 1, the host transmitting/receiving block of I/O unit A1 is connected to the CNC and the host transmitting/receiving block of I/O unit A2 as described above. In addition, a mutual connection is made between the host transmitting/receiving blocks of I/O units A2 and A3, between the host transmitting/receiving blocks of I/O units A3 and A4, and between the host transmitting/receiving blocks of I/O units A4 and A5.
Reference characters B1 to B3 indicate external units (such as robots or machine tools other than M1 to M5) connected to the CNC; signals from these external units may also be transmitted to each of the I/O units A1, A2, . . . through the CNC. These signals are also a type of external signal. In general, external signals are any automated machine control signals other than servo control signals. As is well known, signals (such as servo control signals and feedback signals) for servo control of each axis of the automated machines M1 to M5 are transmitted to and received from the automated machines M1 to M5 directly, bypassing the I/O units A1 to A5.
With the connections shown in FIG. 1, during normal communication (when no diagnosis is being made), external signals generated inside the CNC or passed from an external signal source (one of the external units B1 to B3) to the CNC are forwarded to the I/O units A1, A2, A3, A4, and A5 in sequence; each external signal is selectively received by the host transmitting/receiving block of the I/O unit that needs the external signal, and the received external signal is output to the corresponding machine tool of M1 to M5 through the output line.
An external signal output from one of machine tools M2 to M5 to the corresponding one of I/O units A2 to A5 is forwarded in sequence through the other I/O units on the CNC side to the CNC. An external signal output from machine tool M1 to the corresponding I/O unit A1 is directly transmitted to the CNC.
As shown in this example, when a plurality of I/O units are used, hitherto it has been necessary to install a diagnostic circuit of the type described above in each I/O unit to make the necessary operational checks of the I/O unit. That is, in the prior art a diagnostic circuit is required for each I/O unit, regardless of the number of I/O units, so the system becomes complicated and expensive.
A simple solution to this problem is not to be found in the known literature. Japanese Patent Application Laid-Open No. 2005-25260 proposes technology to provide security for a unit used to input or output safety signals, but the proposed technology is a redundant signal transmission circuit that provides security in case of a local failure; no technology is disclosed that diagnoses I/O units without installing a diagnostic circuit in each of the I/O units.